It is desirable to combine multiple laser beams in an optical writing head to increase the speed of writing an image or document on a print medium. Various techniques are known in the prior art for achieving such result.
U.S. Pat. No. 4,634,232 (M. Tateoka), issued on Jan. 6, 1987, discloses a light source device for providing close spacing of two light beams. In the device, first and second semiconductor lasers are polarized with a predetermined polarization and directed in a plane orthogonal to each other. Optical means capable of rotating the plane of polarization of an incident beam by approximately 90 degrees is disposed between one of the semiconductor lasers and a polarizing beam splitter having an polarization interference film therein. The first and second semiconductor lasers are disposed so that a laser beam from the first laser is polarized so that it is transmitted through the interference film in the polarizing beam splitter while the second laser beam is polarized so that it is reflected by the interference film of the polarizing beam splitter. After passing through the polarizing beam splitter, cross-sectional major and minor directions of the first beam are coordinated with the major and minor directions, respectively, of the second beam.
Various techniques have been provided for combining more than two laser beams in an optical writing head. For example, U.S. Pat. No. 4,743,091 (D. Gelbart), issued on May 10, 1988, discloses a two-dimensional laser diode array for use in optical data storage. The array comprises rows and columns of laser diodes, each diode including a separate collimating lens. The array is imaged onto an optical recording medium which is moving relative to the array image to provide scanning. Since the distance between the individual laser diodes is large, the apparent distance between the array and a scanning lens has to be large. A reverse telescope is used between the array and the scanning lens to significantly reduce this distance. To further reduce this distance, the laser diodes are made to appear closer at the recording medium than their mechanical separation at the array by staggering the laser diodes in the array in a direction perpendicular to the scanning direction (skewing the rows).
U.S. Pat. No. 4,978,197 (K. Horikawa), issued on Dec. 18, 1990, discloses a beam-combining laser beam source device. The device comprises first and second laser beam source sections, and a beam combining optical element such as a polarization beam splitter. Each beam source section includes laser beam sources, a collimator optical system, and optical path adjusting elements. Each collimator optical system comprises lens groups corresponding to the laser beams sources, and a common lens positioned so that the laser light beams radiated from the optical path adjusting elements, along optical paths parallel and close to one another, impinge the common lens. All common lenses are movable along first directions parallel to their optical axes and along second directions normal to the optical axes. Laser light beams from the first and second laser beam source sections are moved along first directions normal to each other and combined by movement of the common lenses along second directions. For a similar arrangement, see U.S. Pat. No. 4,986,634 (K. Horikawa et al.), issued on Jan. 22, 1991.
U.S. patent application Ser. No. 749,037, U.S. Pat. No. 5,258,776 (T. Mackin et al.), filed on Aug. 23, 1991, discloses a thermal printer which includes a thermal print head mounting a plurality of N thermal heating devices such as lasers or resistive heating elements. In the thermal printer, a receiver member is mounted on a rotating drum with a dye carrier member engaging the outer surface of the receiver member in a dye frame image printing area. The thermal heating devices are aligned at a predetermined acute angle theta to a line normal to the rotation of the drum. During high speed rotation of the drum, the thermal heating devices are selectively energized by micropixel clock pulses which are synchronized to the rotational speed of the drum. During each rotation of the drum, N columns of micropixels are printed on the receiver member. Additionally, the energizing of each of the thermal heating elements is timed using the micropixel clock pulses so as to print corresponding micropixels of the N columns of micropixels in a line normal to the rotation of the drum. A requirement to align the line of thermal heating devices at the acute angle theta can result in tight tolerances on the mechanical spacing of the thermal heating devices in a lateral direction, depending on the type of media, modulation scheme, and resolution. Additionally, there must be a time delay between the modulation of the thermal heating devices to account for the offset in scan direction due to the rotation angle being non-zero. A problem with providing more than two laser diodes which are aligned and rotated by the acute angle theta is that a tight tolerance on the mechanical spacing of the laser diodes is necessary in a lateral direction depending on the type of print medium, modulation technique, and resolution used.
It is desirable to provide techniques for combining multiple laser beams to form multiple spot optical heads which reduces mechanical tolerances on the spacing of the laser diodes in the lateral direction.